It is common practice in medical applications in modern science to use high-energy radiation beams for the treatment of certain human medical problems, notably malignant tumors. In utilizing high energy radiation beams on human patients it is critically important that the radiation beam be properly focused so that only the deleterious tissue is harmed. Therefore it has become the practice to develop instruments to analyze the spread and depth penetration of radiation beams so as to more clearly define what portions of human bodies placed in such beams are affected by incident high energy radiation.
It is the usual practice to map the distribution of radiation dose by putting a simulated human body in a radiation therapy device to test the radiation actually applied. Such a simulated human body is often referred to as a "phantom" and is usually filled with either water or a plastic simulating the radiation transmissive properties of the human body. It has also been necessary, however, to provide a sensor capable of accurately measuring the dose which is incident into the phantom. It has therefore been previously done in the art to use flat, pillbox-shaped ion chambers of small size. Such flat chambers are oriented perpendicular to the radiation beam and are used for measuring the dose versus depth relationship in the phantom in cases where the dose gradient is relatively steep, as is true for electron beams. The ion chambers are also used near the front phantom surface in measuring the dosage received from gamma or x-ray radiation beams in the so-called "dose build-up" region.
There exists several commercial models of such ion chambers presently available in the technology. One of the difficulties with such chambers is that they have been previously constructed of various parts which are constructed of diverse materials, having different electron scattering properties. Consequently, when such a chamber is enclosed in a water or plastic phantom and irradiated by an electron beam, the electron flux density is perturbed by the presence of the chamber, leading to erroneous measurements of the ionization or absorbed dose.
One material which has been utilized in various types of radiation phantoms is an epoxy-based plastic manufactured by Radiation Measurements, Inc., Middleton, Wis. and sold under the tradename "SOLID WATER." SOLID WATER is a material which has an x-ray or electron radiation characteristic similar to human body tissue and therefore serves suitably as a phantom. SOLID WATER is sufficiently conductive to allow electrons that reach the end of their range within it to pass easily to ground. On the other hand, it is a poor enough conductor to serve as an effective insulator to conductors placed on the surface of a solid water slab without significant leakage currents occurring between a high voltage electrode and ground.